Adhesives are widely used for various commercial applications such as product assembly and packaging. Two specific types of adhesives are known as pressure sensitive adhesives (PSAs) and hot melt adhesives (HMAs). PSAs are a distinct category of adhesive, which, in dry (solvent free) form, are aggressive and permanently tacky at room temperature and adhere to a variety of surfaces without the need of more than pressure. They do not require activation by water, solvent, or heat. In contrast, HMAs are typically applied to a substrate while in its molten state and cooled to harden the adhesive layer.
Most commercially available HMAs require temperatures of 175° C. or greater to ensure complete melting of all the components and also to achieve a satisfactory application viscosity. While adhesive formulations that can be applied at temperatures below 150° C. can be prepared using low molecular weight components or a high wax content, application viscosity and adhesive properties suffer. While softer or more amorphous components may be added in order to improve adhesion, these components reduce the effective heat resistance.
The use of a HMA with high heat resistance and, in addition, good cold resistance is important in hot filled packaging applications, e.g., sealing and closing operations for cartons, cases or trays used for packaging molten cheese, yogurt or freshly baked goods, which are subsequently subjected to refrigeration or freezing.
PSAs have been used in a variety of applications, as they provide many desirable characteristics such as removability and ease of application. For a more permanent and highly smooth bond, some conventional PSAs may not necessarily have sufficient strength to hold and maintain their adherence on certain substrates. Furthermore, conventional PSA when applied to certain materials, may not be able to withstand exposure to elevated temperatures or high humidity. For example, application of a PSA on acrylic sheets and polycarbonate sheets, known to be “outgassing materials” and difficult to bond, can result in bubbling and delamination.
Curable adhesives (e.g., heat or light-cured) have been used in applications where substrates require substantial permanency and high strength adherence. For optical product applications (e.g., glazings), curable adhesives have been desirable, as they can provide optically clear, strongly adhered laminates (e.g., layered substrates). In addition, paper manufacturers and print manufacturers use PSAs in bookbinding applications, for example.
To achieve both strength and ease of application, hybrid compositions have been developed that can be used in optical applications. For example, a light curable, polyester-based adhesive has been used for plastic glazing applications. In digital video disc (DVD or optical discs) bonding and CRT applications, a liquid adhesive formulation has been used. For bead bonding in making retroreflective articles, a curable polymeric network has been suggested.
Strength and application, however, are not the only criteria that many optical substrates/laminates require. Certain optical products are exposed to harsh environmental conditions, such as heat, UV (solar) light, water, etc. For example, vehicle windshields generally exist in outdoor conditions that submit them to all types of weather. These windshields typically include substrates such as acrylic or polycarbonate, adhered to a solar or infra-red (IR) reflecting film made from a multi-layer optical film (MLOF) (3M Co; St. Paul, Minn.). The materials may become optically obstructed if the adhesion between the layers is damaged or compromised.
One prior art method film-forming polymer and a particulate thermoplastic component selected from the group consisting of thermoplastic polymers, thermoplastic resins, hot melt adhesive compositions, and mixtures thereof is disclosed in U.S. Pat. No. 6,548,579. The thermoplastic component has a mean particle size of from about 10 microns to about 1,500 microns. There continues to be a need in the art for improved adhesives that can be applied at low temperatures, to a variety of substrates, and under a variety of environmental conditions.